This invention relates to a breathable film-forming composition having an organic filler which alleviates die build-up during the extrusion of breathable films, and which reduces the cost associated with inorganic fillers. More specifically, the invention is directed to breathable films utilizing polystyrene filler and a compatibilizer system which effectively compatibilizes and disperses the polystyrene filler in the polymer blend used to make the film.
Vapor permeable, liquid impermeable polymeric films are known in the art. One method of making a polymeric film vapor permeable, involves mixing a matrix polymer with a substantial quantity (e.g., 10-70% by weight) of an organic or inorganic particulate filler such as, for example, calcium carbonate, and extruding a film from the blend. The matrix polymer may include a polyolefin, such as polyethylene or polypropylene, or various olefin copolymers. The film may be a monolayer film, a multilayer film which contains the filled layer as a primary layer, or a multilayer film having more than one filled layer.
Then, the film is heated and stretched, causing voids to form in the areas surrounding the filler particles. The voided film is characterized by thin polymer membranes and/or fine pore networks which permit the molecular diffusion of water vapor through the film, but which block the passage of liquids. In essence, a tortuous path is created from one film surface to the other which permits transfer of vapors but not liquids.
Breathable films are employed in backsheets, for example as a backsheet component laminated to a nonwoven web and/or other layers, in many of today""s personal care absorbent articles. Diapers are one example. Filled, stretched polyolefin films provide good water vapor transmission, making the diapers more comfortable to the wearer. As a result, the relative humidity and temperature within the diaper or other product can be reduced by using breathable films and laminates. Breathable films are also used in surgical gowns. Again, the objective is to provide good water vapor transmission and make the gowns more comfortable to the wearer.
Breathable films are commonly made using inorganic fillers, such as calcium carbonate. Inorganic fillers are relatively easy to disperse in a polyolefin matrix, for instance, and yield films with excellent breathability. Yet calcium carbonate and other inorganic fillers have a disadvantage in that the filler tends to accumulate around the lip of the extrusion die during manufacture of the film. To alleviate this, a center layer filled with calcium carbonate has been coextruded with much thinner layers which contain little or no filler. This approach reduces filler build-up at the die, but often results in a less breathable product because the unfilled skin layers, no matter how thin, are less microporous than the filled core layer. Inorganic fillers are also somewhat expensive, due in part to their high density.
Attempts have been made to make suitable breathable films using organic fillers. Organic fillers can be less expensive to use because they generally have lower densities than inorganic fillers. Organic fillers may also reduce the problem of die build-up. However, organic fillers have a tendency to either agglomerate with each other, or disperse too finely in the polymeric medium upon which the film is based. If the organic filler is incompatible with the polymer used to make the film, the filler particles may agglomerate excessively. If the organic filler is compatible with the film-forming polymer, the filler particles may disperse too finely, and may achieve thermodynamic equilibrium by dissolving in the film. Thus, there is a need or desire for a breathable film whose filler has the advantages provided by an organic filler, and which exhibits a more useful degree of dispersion.
The present invention is directed to a breathable film composition, and a film, containing an organic filler in a matrix polymer, and a compatibilizing system which effectively disperses the filler in the matrix polymer without causing excessive dispersion or dissolution. The present invention also includes a multilayer breathable film utilizing a conventional inorganic film in the core layer, and an organic filler in the skin layers. The films of the invention reduce the die build-up associated with inorganic fillers, without reducing breathability. The films of the invention also reduce the cost associated with using strictly inorganic fillers.
In accordance with the invention, a composition including a thermoplastic polymer, an organic filler, and a compatibilizing system is provided. The thermoplastic polymer may be a polyolefin. The organic filler is preferably incompatible with the thermoplastic polymer, thereby preventing dissolution or excessive dispersion in the polymer matrix. Put another way, the organic filler particles should have a tendency to agglomerate with each other in the polymer matrix, but for the inclusion of a compatibilizing system. The organic filler should also have a higher melting point than the polymer matrix. When the polymer matrix includes a polyolefin, the organic filler may include polystyrene beads.
The compatibilizing system should have a tendency to prevent agglomeration of the organic filler in the polymer matrix, and/or a tendency to break up existing agglomerates of the organic filler. Yet the compatibilizing system should not break down the organic filler into particles which are too fine to sufficiently effect breathability during preparation of the breathable film. Also, the compatibilizing system should not cause dissolution of the organic filler in the polymer matrix. When the polymer matrix includes a polyolefin, and the filler includes polystyrene particles, the compatibilizing system may include a styrene butadiene copolymer. The styrene butadiene copolymer may be included in the polystyrene filler, and may be either chemically reacted to or blended with the polystyrene in the beads. More desirably, the styrene butadiene copolymer may be incorporated into the polymer matrix without forming part of the beads.
The compatibilizing system should be present in an amount sufficient that the organic filler particles are at thermodynamic equilibrium in the polymer matrix in a dispersed particle phase having a mean particle diameter of about 0.1-25 microns. In other words, the thermodynamic equilibrium should not favor a predominance of large, agglomerated particles, and should not favor dissolution of the filler particles, or excessive dispersion which renders the particles too small to effectively promote breathability in a stretched film.
The composition is melt blended and extruded into a film. The film may be a monolayer or multilayer film. The film is then stretched, monoaxially or biaxially using conventional techniques to yield a breathable microporous film.
With the foregoing in mind, it is a feature and advantage of the invention to provide a breathable film-forming composition including a thermoplastic polymer matrix material, an organic filler, and a compatibilizing system which establishes thermodynamic equilibrium between the polymer matrix material and the organic filler with the organic filler in an effective dispersed particulate state.
It is also a feature and advantage of the invention to provide a breathable film constructed from the breathable film-forming composition of the invention.
It is also a feature and advantage of the invention to provide a breathable multilayer film utilizing a conventional filler in the core layer, and utilizing the breathable film-forming composition of the invention in one or both skin layers.
It is also a feature and advantage of the invention to provide a breathable laminate in which the breathable film of the invention is laminated to a nonwoven web.
It is also a feature and advantage of the invention to provide a breathable garment, such as a breathable diaper or surgical gown, which utilizes the breathable film of the invention in a laminate.
The foregoing and other features and advantages will become further apparent from the following detailed description of the presently preferred embodiments, read in conjunction with the accompanying drawings. The detailed description and drawings are intended to be illustrative rather than limiting, the scope of the invention being defined by the appended claims and equivalents thereof.